1. Field of the Invention
The present invention relates to wrought products and structural members, made of an aluminium alloy, in particular for aircraft construction. Wrought products can be rolled products (such as thin sheets, medium sheets, thick sheets), extruded products (such as bars, extruded profiles, or tubes), and/or forged products.
2. Description of Related Art
Intelligent structures have shown that they were capable of having a broad range of applications likely to increase performance levels in the field of the aerospace industry. The information gathered by sensors built into the structure can have numerous applications linked both to flight as well as to the design or maintenance of devices.
In particular, the inclusion of sensors in structural members offers not only potential for improving how the health of a structure is monitored, but can also open up new design possibilities.
Thus, structural analysis of damage tolerance generally involves evaluating the number of stress cycles or peaks that the structure can withstand between the moment when a defect is detectable and the moment when this defect is sufficiently large to be deemed critical. Criticality can be judged on the basis of a calculation of instability, with the material R-curve according to ASTM E561, or on the basis of a judgment such as “the crack must not exceed two inter-smooth spaces.” The number of stress peaks thus calculated, or number of flights for an aircraft structure, should be less than or equal to the anticipated inspection interval for the structure.
The detectable defect is often deemed as being one that can be visually detected. In a stiffened panel, it frequently comprises a crack in the skin of a few dozen millimeters, on either side of a stiffener which is itself cracked. Such being the case, the latter hypothesis is very detrimental to calculations. As a matter of fact, the load borne by the stiffener, due to the fact that it is presumed to be cracked, is transferred onto the skin which comprises the crack. The stress intensity factor applied to the crack is therefore significantly increased. Therefore, in order to ensure the anticipated inspection interval, one is led to increase the skin thickness, with a detrimental effect on the weight of the panel which can be estimated at 20%.
The addition of a sensor indicating whether the stiffener is broken or not would therefore contribute a weight gain on the order of magnitude mentioned above. This sensor can operate according to several physical principles: vibrations, currents and/or transmission of light.
The incorporation of a sensor into a metal structural member is difficult, in particular because of the risk of damaging the sensor or the structural member during manufacture.
It has been proposed to affix a sensor to the surface of the structural member.
U.S. Pat. No. 4,636,638 discloses the adhesive bonding of an optical fiber to the surface of a structural member in proximity to the primary sources of stress.
U.S. Pat. No. 5,525,796 discloses an improvement in the preceding method of the '638 patent, in which the optical fiber, surrounded by a metal sheath, is welded onto the surface of the structural member.
CA Patent Application No 2 334 051 discloses a method and a system for detecting temperature and mechanical strain by using a Bragg grating optical fiber deposited on a substrate and protected by a protective layer.
The attachment of the sensor to the surface of the structural member poses numerous problems: the sensor is sensitive to defects detected only on the surface, and it can be damaged accidentally. Furthermore, attaching the sensor to the surface requires steps that are long and costly, because they must be carried out during the final steps of manufacturing the aeroplane, in particular after surface treatment steps.
U.S. Pat. No. 5,283,852 proposes to incorporate an optical fiber into a protective tube when casting the metal. However, the deformation of the metal during the steps for deforming the casting, which are necessary for manufacturing the structural member, are likely to break the fiber, even in the presence of a protective tube. Furthermore, a fiber incorporated into a protective tube, which is not in direct contact with structural member, is not very sensitive to the stresses, or even to the fractures that the latter undergoes.
U.S. Pat. No. 6,685,365 proposes placing optical fibers between two sheets of aluminium and assembling them at a low temperature. This type of method, intended for the manufacture of optical cables, is not suited to the construction of structural members because the mechanical properties of the assembly are insufficient.